Identifying people with high skin cholesterol is useful in determining individuals at risk of either having or developing atherosclerosis as well as those at risk of having similar or other diseases attributable to high cholesterol levels.
Coronary artery disease caused by atherosclerosis remains the number one cause of morbidity and mortality in North America and many other parts of the world. Prevention and intervention requires the cost effective identification of those individuals not only having the disease but also those at risk of developing the disease. Therefore, there is currently much interest in determining levels of marker molecules that are able to predict risk of atherosclerotic disease.
Measurement of blood plasma total cholesterol levels is one of the most widely used methods to determine risk of atherosclerosis. However, plasma total cholesterol levels alone do not accurately predict risk and better results have been obtained through measurement of plasma lipoproteins. Measurement of cholesterol in both low density lipoprotein (LDL) and high density lipoprotein (HDL) show advantages over measuring total cholesterol levels. All of these measurements require blood sampling after a long period of fasting so that dietary cholesterol does not interfere. Additionally, the sampling can be uncomfortable and carries some small risk of infection. Furthermore, the analysis often requires complicated and expensive equipment.
One example of a device that allows a user to obtain a blood cholesterol level is shown in U.S. Pat. No. 5,340,539. This patent circumvents some of the problems associated with visits to a doctor or clinic and makes fasting more convenient. However, a blood sample obtained from a finger prick with a lancet device is still required, which can be objectionable to many individuals.
In many cases, however, the levels of plasma cholesterol and lipoproteins do not correlate with the extent of atherosclerotic disease. It is therefore desirable to assay other marker molecules that reflect the extent of atherosclerosis and provide a risk assessment of cardiovascular disease.
For example, significant amounts of cholesterol occur in tissue in addition to that found in plasma and increased levels in tissue have been shown to play a major role in development of atherosclerosis. It has been demonstrated that the accumulation of cholesterol in tissues, including the skin, correlates closely with the amount of cholesterol found in arterial wall deposits. The measurement of cholesterol in skin, therefore, may reflect the extent of atherosclerosis. Indeed, cholesterol levels in skin biopsy samples have been shown to correlate with arteriosclerosis and to provide a risk assessment for patients with ischemic cardiac disease (Bouissou H., De Graeve J., et al. Ann. Biol. Clin. Vol 40, 364-365, 1982). Also, measurement of cholesterol extracted from lyophilized skin biopsy samples correlates with serum lipid quotient in normals and in patients with ischemic cardiac disease (Y. P. Nikitin et al. Kardiologiia, II, 48-51, 1987).
A drawback of obtaining skin biopsy samples for skin cholesterol determinations for risk assessment of atherosclerosis is that there can be pain in obtaining the skin samples. Moreover, a risk of infection at the biopsy site is possible. Obtaining the skin samples typically requires trained professionals. In addition, such samples contain subcutaneous fat and several layers of skin, some of which are highly vascularized. Consequently these samples contain heterogeneous sources of cholesterol and do not give reproducible and reliable cholesterol assay results.
One method for assaying various substances in the blood directly below the surface of the skin or on its surface is described in U.S. Pat. No. 4,458,686. This patent is based on electrochemical measurement of generated oxygen concentrations. For non-volatile substances that do not diffuse through the skin, however, it is necessary to implant enzymes under the skin to effect oxygen changes at the skin surface. The patent discloses the use of the enzyme cholesterol oxidase to determine cholesterol but it appears that it is blood cholesterol rather than skin cholesterol that is being measured.
Consequently, there is a need for a simple method and apparatus for non-invasive measurement of cholesterol in the skin that is unaffected by other sources of cholesterol.